It is known that polyester fibers for industrial applications are in most cases subjected to high mechanical and or thermal stressors in use. In addition, there are in many cases stressors due to chemical and other ambient influences, to which the material has to offer adequate resistance. As well as adequate resistance to all these stressors, the material has to possess good dimensional stability and constancy for its stress-strain properties over very long use periods. Nor may an electrostatic charge build up on the material during processing and in use.
One example of industrial applications comprising a combination of high mechanical, thermal, chemical and electrical stresses is the use of monofilaments in filters, screens or as conveyor belts. This use requires monofilaments having excellent mechanical properties, such as high initial modulus, breaking strength, knot strength, loop strength and also high abrasion resistance coupled with high hydrolysis resistance in order that they may withstand high stresses encountered in their use and in order that the screens or conveyor belts may have an adequate use life.
Industrial manufacturers, such as paper makers or processors, utilize filters or conveyor belts in operations taking place at elevated temperatures and in hot moist environments. Polyester-based manufactured fibers have a proven record of good performance in such environments, but when used in hot moist environments polyesters are vulnerable to mechanical abrasion as well as hydrolytic degradation.
Abrasion can have a wide variety of causes in industrial uses. For instance, the sheet-forming wire screen in papermaking machines is in the process of dewatering the paper slurry pulled over suction boxes, and this results in enhanced wear of the wire screen. At the dry end of the papermaking machine, wire screen wear occurs as a consequence of speed differences between the paper web and the wire screen surface and between the wire screen surface and the surface of the drying drums. Fabric wear due to abrasion also occurs in other industrial fabrics, for instance in transportation belts due to dragging across stationary surfaces, in filter fabrics due to the mechanical cleaning and in screen printing fabrics due to the movement of a squeegee across the screen surface.
The forming wire screens of state of the art papermaking machines utilize multi-ply woven fabrics. To maximize the speed of dewatering the paper, suction boxes are utilized on the wire screen underside to speed paper web dewatering by means of underpressure. The contact surfaces of the edges of these suction boxes with the forming fabric consist in general of ceramic to prevent excessive wear of the suction boxes.
On the other hand, the high manufacturing speeds, the rubbing due to the fillers added to the monofils and the sucking effect of the papermaking machine lead to high wear on the underside of the multi-ply forming fabric.
Monofilaments made of nylon, for example nylon-6 or nylon-6,6, are still being used to improve the abrasion resistance of the wire screen underside. This is where it is predominantly monofilaments made of polyethylene terephthalate (hereinafter PET) which are used because of their higher dimensional stability, and it is of them that the forming wire screen fabric consists essentially. One tried and tested construction for the wire screen underside is that of an alternating weft in which a backing weft of a nylon monofil alternates with a backing weft of PET monofil. This results in a compromise of abrasion resistance and dimensional stability.
The higher water imbibition of nylons compared with PET leads to lengthening of the weft threads in operation of the wire screen. As a result, the wire screens are prone to the undesirable effect known as edge curling in that they curl up at the edges and no longer lie flat within the papermaking machine.
There have been numerous attempts to replace nylon monofilaments with monofilaments made of other abrasion-resistant polymers that have a low water imbibition as well as being deformation resistant.
An example is monofilaments made of PET blends admixed with 10-40% of thermoplastic polyurethane (TPU) (cf. for example EP-A-387,395). Similarly, mixtures of thermoplastic polyester, for example polyethylene terephthalate isophthalate, and thermoplastic polyurethane having melting points of 200 to 230° C. have been used (cf. for example EP-A-674,029).
The prior art further comprises monofilaments having a core-sheath structure in each of which the sheath consists of a mixture of thermoplastic polyester having a melting point of 200 to 300° C., for example PET, and of thermoplastic elastomeric copolyetherester having selected polyetherdiol building block groups as soft segments, that likewise exhibit improved abrasion resistance (cf. for example EP-A-735,165).
Further polyester compositions comprising crystalline thermoplastic polyester resins, polyester elastomers and sorbitan esters are known from DE 691 23 510 T2. These are notable for good moldability, in particular for good releaseability.
DE 690 07 517 T2 discloses polyester compositions comprising an aromatic polycarbonate, a polyester derived from alkanediol and benzene-dicarboxylic acids, and a polyesterurethane elastomer or a polyether imide ester elastomer. These combine improved flow properties with good mechanical properties.
While these prior art strands do provide adequate abrasion resistance, electrical conductivity still leaves something to be desired in many cases. True, it has long been known that carbon black can be incorporated in strands to improve electrical conductivity. However, prior art solutions typically only provide electrical conductivities of up to 10−6 siemens/cm. When prior art carbon blacks are used to enhance electrical conductivity, it is found that when the strands produced are drawn the conductive paths formed by the carbon black are interrupted, and that a distinct reduction in electrical conductivity occurs as a result.
WO-A-98/14,647 describes an attempt to remedy this disadvantage by producing a sheath-core filament comprising a sheath polymer having a lower melting point than the core polymer. After drawing, the sheath is incipiently melted, so that the strand shrinks and interrupted bridges of electrically conductive material can re-form. This does indeed push electrical conductivity back up; however, the thermal treatment leads to a decrease in the degree of orientation of the molecular chains and hence to a reduction in the strength of the filament.
EP-A-1,559,815 describes coating a ready-produced strand with a mixture of carbon nanotubes and a polymer. Since the coated strand is not further stretched, the carbon bridges in the amorphous coating are not ruptured, which results in very good electrical conductivities.